Factor H-binding protein (fHbp) is part of two vaccines in clinical development for prevention of group B meningococcal disease for which currently there is no licensed vaccine. Meningococci use fHbp to bind complement factor H (fH) to evade host immunity; binding is specific for human fH. Antibodies to fHbp can inhibit binding of fH and increase susceptibility of the organisms to complement-mediated bacteriolysis. Our preliminary studies in newly created human fH transgenic mice showed that binding of human fH to fHbp decreased the protective antibody responses. A single amino acid substitution in fHbp eliminated fH binding and increased the protective responses. In Aim 1 we will use random mutagenesis to identify and investigate the vaccine potential of additional mutants that eliminate fH binding. Sequence variants of fHbp are classified in two antigenic sub-families; the mutation that eliminated fH binding in one sub-family had no effect in the other. Therefore in Aim 2 we will use site-specific and random mutagenesis to generate mutants in the fHbp from the other sub- family. Binding of fH to fHbp may create neoantigens capable of eliciting autoantibodies to fH. In Aim 3, we will test whether the native fHbp that binds human fH elicits autoantibodies in transgenic animals, which would provide a strong rationale for the use of fH non-binding fHbp mutants as vaccines. By making relatively minor modifications to fHbp antigens already in clinical development, the proposed studies will illuminate the relationship between binding of fH to a vaccine antigen and vaccine safety and efficacy.